xenconsoled: portability fixes:
- Use openpty(), which does the same as the sequence
of open(), grantpt(), unlockpt(), ptsname(), tcgetattr()
simplifies code
- Check return code from tcsetattr()
- sprintf() -> snprintf()
- OpenBSD lacks POSIX grantpt() and unlockpt()
requires use of openpty()
- Solaris lacks POSIX openpty() via feedback from SUN (John Levon)
implement openpty() for Solaris, tested and ok'd by SUN (John
Levon)

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>